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Author(s): J. R. Wible (corresponding author) [1]; G. W. Rougier [2]; M. J. Novacek [3]; R. J. Asher [4] Estimates of the time of origin for placental mammals from DNA [...]

A massive effort to sample mammals for genes has yielded new proposals for the branching architecture of the great radiation of placental mammals. Some of these are notably discrepant with morphologically based analyses, but they suggest new research that should address several major outstanding issues.

The current massive degradation of habitat and extinction of species is taking place on a catastrophically short timescale, and their effects will fundamentally reset the future evolution of the planet's biota. The fossil record suggests that recovery of global ecosystems has required millions or even tens of millions of years. Thus, intervention by humans, the very agents of the current environmental crisis, is required for any possibility of short-term recovery or maintenance of the biota. Many current recovery efforts have deficiencies, including insufficient information on the diversity and distribution of species, ecological processes, and magnitude and interaction of threats to biodiversity (pollution, overharvesting, climate change, disruption of biogeochemical cycles, introduced or invasive species, habitat loss and fragmentation through land use, disruption of community structure in habitats, and others). A much greater and more urgently applied investment to address these deficiencies is obviously warranted. Conservation and restoration in human-dominated ecosystems must strengthen connections between human activities, such as agricultural or harvesting practices, and relevant research generated in the biological, earth, and atmospheric sciences. Certain threats to biodiversity require intensive international cooperation and input from the scientific community to mitigate their harmful effects, including climate change and alteration of global biogeochemical cycles. In a world already transformed by human activity, the connection between humans and the ecosystems they depend on must frame any strategy for the recovery of the biota.

We describe here two new specimens of the mammal Deltatheridium pretrituberculare from the Late Cretaceous period of Mongolia. These specimens provide information on tooth replacement in basal therian mammals and on lower jaw and basicranial morphology. Deltatheroidans, known previously from isolated teeth, partial rostra and jaws from the late Cretaceous of Asia and possibly North America, have been identified variously as eutherians, as basal metatherians (the stem-based clade formed by marsupials and their extinct relatives), or as an outgroup to both eutherians and metatherians. Resolution of these conflicting hypotheses and understanding of the early evolution of the therian lineage have been hampered by a sparse fossil record for basal therians. The new evidence supports metatherian affinities for deltatheroidans and allows a comprehensive phylogenetic analysis of basal metatherians and marsupials. The presence of specialized marsupial patterns of tooth replacement and cranial vascularization in Deltatheridium and the basal phylogenetic position of this taxon indicate that these features are characteristic of Metatheria as a whole. Other morphological transformations recognized here secure the previously elusive diagnosis of Metatheria. The new specimens of Deltatheridium illustrate the effectiveness of fairly complete fossil specimens in determining the nature of early evolutionary events.

Suprising new fossils - a skeleton and a jaw - give us a much clearer picture of mammals that lived during the time of non-avian dinosaurs; the new finds illuminate the early evolution of the lineage leading to modern mammals, and challenge traditional understanding of placental mammal evolution and biogeography.

An important transformation in the evolution of mammals was the loss of the epipubic bones. These are elements projecting anteriorly from the pelvic girdle into the abdominal region in a variety of Mesozoic mammals, related tritylodonts, marsupials and monotremes but not in living eutherian (placental) mammals. Here we describe a new eutherian from the Late Cretaceous period of Mongolia, and report the first record of epipubic bones in two distinct eutherian lineages. The presence of epipubic bones and other primitive features suggests that these groups occupy a basal position in the Eutheria. It has been argued that the epipubic bones support the pouch in living mammals, but epipubic bones have since been related to locomotion and suspension of the litter mass of several attached, lactating offspring. The loss of the epipubic bones in eutherians can be related to the evolution of prolonged gestation, which would not require prolonged external attachment of altricial young. Thus the occurrence of epipubic bones in two Cretaceous eutherians suggests that the dramatic modifications connected with typical placental reproduction may have been later events in the evolution of the Eutheria.

Some shocking revelations in the fossil record--including fossilized hair as well as teeth and skeletons--both illuminate and complicate views evolution of ancient mammals that lived during, and just after, the age of the dinosaurs.

An embryonic skeleton of a nonavian theropod dinosaur was found preserved in an egg from Upper Cretaceous rocks in the Gobi Desert of Mongolia. Cranial features identify the embryo as a member of Oviraptoridae. Two embryo-sized skulls of dromaeosaurids, similar to that of Velociraptor, were also recovered in the nest. The eggshell microstructure is similar to that of ratite birds and is of a type common in the Djadokhta Formation at the Flaming Cliffs (Bayn Dzak). Discovery of a nest of such eggs at the Flaming Cliffs in 1923, beneath the Oviraptor philoceratops holotype, suggests that this dinosaur may have been a brooding adult.

The fossil record offers the only direct evidence of extinct life and thus has figured prominently in considerations of evolutionary patterns. But the incomplete nature of the fossil record has also been emphasized in arguments that fossils play only a secondary role in the recovery of phylogenetic histories based on extant taxa. Although these criticisms recently have been countered, there is no general understanding of the correspondence between the fossil record and phylogeny. An empirical survey of recently published studies suggests no basis for assuming that the stratigraphic occurrence of fossils always provides a precise reflection of phylogeny. Nevertheless, our survey of a sample of taxa shows a tendency for positive correlation between age and clade rank and, hence, a degree of correspondence between phylogenetic pattern and the paleontologic record.

Recent palaeontological discoveries and the correspondence between molecular and morphological results provide fresh insight on the deep structure of mammalian phylogeny. This new wave of research, however, has yet to resolve some important issues.

The erinaceomorph insectivorans, which include the living hedgehog and a diversity of archaic taxa, have long been considered to be close relatives or direct ancestors of primates and a variety of other placental mammal orders. We report here on the oldest known erinaceomorph basicranium which provides new evidence against the view that primates and erinaceomorphs share a close common ancestry.

The earliest-known bats are represented by excellent fossil material, including virtually complete skeletons of Icaronycteris index from the early Eocene (50 Myr BP) of western Wyoming and Palaeochiropteryx tupaiodon from the middle Eocene (45 Myr BP) 'Grube Messel' of western Germany. These taxa have been closely allied with Recent Microchiroptera, a suborder of diverse bats noted for their powers of ultrasonic echolocation. A problem with this relationship is the alleged absence in the Eocene forms of specializations in the auditory region and other aspects of the skeletal system. It has been proposed, therefore, that the oldest bats are members of a group more primitive and possibly ancestral to the Microchiroptera and the visually oriented Megachiroptera. Previously undescribed specimens now show, however, that Icaronycteris and Palaeochiropteryx share special basicranial features with microchiropterans which suggest comparable refinement of ultrasonic echolocation. These results support the theory that a sophisticated sonar system was present in the earliest records of microchiropteran history.